Glaze defects
Glaze defects are any flaws in the surface quality of a ceramic glaze, its physical structure or its interaction with the body. Body/glaze interaction problems Glaze defects can be as a result of the incompatibility of the body and the selected glaze, examples including Crazing and Peeling. Crazing Crazing is a spider web pattern of cracks penetrating the glaze. It is caused by tensile stresses greater than the glaze is able to withstand”Ceramics Glaze Technology.” J.R.Taylor, A.C.Bull. Institute Of Ceramics / Pergamon Press. 1986.Ceramic Glazes. 3rd edition. Parmelee C. W. The Maple Press Company. 1973. Common reasons for such stresses are: a mismatch between the thermal expansions of glaze and body; from moisture expansion of the body; and in the case of glazed tiles fixed to a wall, movement of the wall or of the bonding material used to fix the tile to the wallDictionary Of Ceramics. Arthur Dodd & David Murfin. 3rd edition. The Institute Of Minerals. 1994.. The cracks can allow the ingress of water, dirt and bacteria into the cracks. Once fired ware tends to be more resistant to crazing due to better development of the glaze/body interfacial layer, which reduces stress gradients between the glaze and bodyDictionary Of Ceramics. Arthur Dodd & David Murfin. 3rd edition. The Institute Of Minerals. 1994.. '' The causes of crazing include:”Ceramics Glaze Technology.” J.R.Taylor, A.C.Bull.'' Institute Of Ceramics / Pergamon Press. 1986.Dictionary Of Ceramics. Arthur Dodd & David Murfin. 3rd edition. The Institute Of Minerals. 1994.. Thermal expansion mis-match between the glaze and the body. Poor glaze/body fit is the main cause of crazing and can be due to: *Under-firing (earthenware body types) resulting in failure to develop sufficient body thermal expansion. *Firing too quickly, resulting in failure to achieve sufficient heatwork. *Low thermal expansion body. *High thermal expansion glaze. *Over-firing of vitreous ware. Moisture expansion of the body - porous bodies will swell slightly due to absorption of moisture. Where glazes are in only slight compression this can be sufficient to bring them into tension. The problem results in delayed or secondary crazing which occurs over a period of time after the ware has been produced. Glazing too thickly. This is a common cause of crazing. Glazes, which should be craze resistant, can craze if applied too thickly. This is because the further the glaze surface is away from the body the lower the compression acting on it. Thermal shock. Opening the kiln too soon above 100°C can cause crazing and dunting. Above 200°C catastrophic failure can occur due to the volume changes at the cristobalite inversion (around 225°C) Glaze Fit. The matching of the thermal expansion of a glaze to that of the body on which it is held. To prevent crazing the glaze must be in compression when the ware has been cooled from the kiln to room temperature; to achieve this, the thermal expansion of the glaze must be less than that of the body. Steger's Crazing Test is a method for the assessment of the glaze fit. It is undertaken by measuring any deformation on cooling of a thin bar that was glazed only on one sideW. Steger. Ber. Deut. Keram. 9 (4). 1928. “Evaluation Of The Steger Method In The Determination Of Ceramic-Glaze Joining.” M.Peterson, A.M.Bernardin, N.C.Kuhnen, H.G.Riella. Materials Science and Engineering A 466 (2007). Pg.183–186"Temperature Gradient Method for Determining Firing Range of Ceramic Bodies." R. Stone. Journal of the American Ceramic Society. Volume 36 Issue 4, Pg. 140 - 142. 2008.. A common method of testing glazed ceramic ware for crazing resistance is to expose pieces of ware to the steam in an autoclave at a minimum of 50 p.s.i.British Standard 3402 and British Standard 4034"Autoclave Test Results On Crazing Of Sanitary Ware And Wall Tiles." Journal of the American Ceramic Society. Volume 29 Issue 7, Pages 203 - 204. 2010. Seger's Rules are a series of empirical rules put forward by the H.A.Seger for the prevention of crazing and peeling. To prevent crazing, the body should be adjusted as follows: decrease the clay, increase the free silica; replace some of the ball clay by kaolin; decrease the feldspar; grind the silica more finely; biscuit fire at higher temperature. Alternatively, the glaze can be adjusted: increase silica and/or decrease fluxes; replace some SiO2 by B2O3; replace fluxes of high equivalent weight by fluxes of lower equivalent weight. To prevent peeling, the body or glaze should be adjusted in the reverse direction”The Collected Writings Of Hermann August Seger.” H.A.Seger, E.Cramer. Nabu Press. 2010 POD edition.. Peeling The breaking away of glaze from ceramic ware in consequence of too high a compression in the glaze layer; this is caused by the glaze being of such a composition that its expansion coefficient is too low to match that of the body. It is the opposite of crazing, as are the preventative steps: see Seger's Rule above. Peeling is also known as shivering."Dictionary Of Ceramics. Arthur Dodd & David Murfin. 3rd edition. The Institute Of Minerals. 1994."Glazing Faults In Raw Glazes". Ceram. Inf. 18, (207), 348, 1983."Ways Of Eliminating Glaze Defects In Practice. Pt.1. Crazing And Peeling." H Simonis. Keram. Z. 31, (12), 717, 1979. Metal release Regulations have existed since the late 1960s to protect consumers from the potential risk of toxic materials, mainly metals, being released from glazes into drink and foodstuffs. Lead and cadmium are the metals of greatest concern, although testing can be extended to include others. The propensity for any glaze to release metal may depend on complex interactions between the formulation used, any applied decoration and the kiln atmosphere”Ceramics Glaze Technology.” J.R.Taylor, A.C.Bull. Institute Of Ceramics / Pergamon Press. 1986.. Monitoring the level of metal release from glazed ware forms part of the Quality control procedures of all reputable producers"Countermeasures For Lead Release From On-Glaze Decorating Of Porcelain Products." A.Fukunaga. Ceram.Jap. 37, No.8. pg.628. 2002."Ceramics Glaze Technology." J.R.Taylor, A.C.Bull. Institute Of Ceramics / Pergamon Press. 1986.. Test methods are specified according national and international standards, although testing usually involves: the ware being immersed or filled with a 4% acetic acid solution; covered and left for 24 hours at room temperature, although if cooking ware is being tested higher temperatures are needed; the acetic acid solution decanted from the ware and the concentration of leached metal measured by Atomic absorption spectroscopy"Whitewares: Production, Testing And Quality Control." W.Ryan, C.Radford. Pergamon Press / Institute Of Ceramics. 1987.. Acceptance limits are enforced by legislation, and whilst varying between countries all are within the ppm range. Some of the most well recognised legislation are: across Europe 'EC Directive 84/500/EEC 1984'; for the UK 'GB Ceramic Ware (Safety) Regulations SI 1647, 1988'; and for the USA 'FDA Compliance Policy Guide 7117.06 and 7117.07 for cadmium and lead.'"Decoration Trends In The Chinese Tableware Market." H.Zhou. Ceramic Forum International. DKG 85,No.5,2008,p.E30-E31."Understanding Heavy Metal Limits." Ceramic Industry. 158,No.4,2008,p.36-38."Toxic Metal Release From Ceramic Tableware: A Guide To Worldwide Regulations." J. Dawson. Ceram Research. Special Publication No.145, pg.18. 2007. Glaze Surface Defects Blisters A large bubble sometimes present as a fault in ceramic ware. Blisters appear as large bubbles either just below or penetrating the surface, leaving sharp, rough edges which collect dirt. The surface of the glaze is very unpleasant and looks like a boiled mass of bubbles, craters and pinholes"Dictionary Of Ceramics." Arthur Dodd & David Murfin. 3rd edition. The Institute Of Minerals. 1994."Formation Of Blisters And Pinholes In Raw Glazes Of Ceramic Sanitary-Ware. Pt.1. Literature Review On Their Formation And Determination ." G.Schopwinkel, H.W.Hennicke. Keram.Z. 41, No.11, pg.830-834. 1989.. Crawling A defect that appears as irregular, bare patches of fired body showing through the glaze where it has failed to adhere to or wet the body on firing. The cause is a weak bond between glaze and body; this may result from greasy patches or dust on the surface of the biscuit ware or from shrinkage of the applied glaze slip during drying. The fault is more likely to occur with once-fired ware such as sanitaryware."Dictionary Of Ceramics. Arthur Dodd & David Murfin. 3rd edition. The Institute Of Minerals. 1994.”Ceramics Glaze Technology.” J.R.Taylor, A.C.Bull. Institute Of Ceramics / Pergamon Press. 1986." "Crawling" - A Serious Defect In Glazes After Firing." S.K.Mukherji, B.B.Machhoya, R.M.Savsani, T.K.Dan. Ceramurgia 23, No.6, pg. 269-275. 1993''Problem Of The Glazing Defect Crawling.'' H.Mortel, I.Berger. Keram.Z. 45, No.5, pg. 259-262. 1993.. Metal Marking Metal marks are dark lines, often accompanied by damage in the glaze, caused by the deposition of metal during the use of metal utensils. The cutlery, or other relatively soft metal, will leave a very thin smear of metal on pottery ware if the glaze is minutely pitted. A glaze may have this defective surface as it leaves the glost kiln, or it may subsequently develop such a surface as a result of inadequate chemical durability. The fault is also known as cutlery marking"Dictionary Of Ceramics. Arthur Dodd & David Murfin. 3rd edition. The Institute Of Minerals. 1994."The Marking of Glazes by Cutlery." W.P.Howells, W.Roberts. British Ceramic Research Association. Technical Note 98. 1966."Metal Marking Of Dinnerware." C.Prieur, G.Bisson, C.Casset. Industrial Ceramics No.927, pg.458-461. 1997."Testing For Metal Marking Resistance." Z.C.Seedorff, R.C.Patterson, H.J.Pangels, R.A.Eppler. Ceram.Eng.Sci.Proc. 13, No.1/2, pg. 196-209. 1992.. Pin-hole A fault that is commonly the result of a bubble in the glaze when it was molten that burst but was only partially healed. The bubbles are most often from gas that originates from air trapped between the particles of powdered glaze as the glaze begins to mature, or from gases evolved from carbonate compounds"Practical Suggestions For Control Of Glaze Defects. Pt.2." H.Simonis. Keram. Z. 32, (10), 593, 1980.Pinholing in Sanitary Whiteware. E G Walker. The British Ceramic Research Association. TN20. 1962.. A specific example of pin-holes is Spit-out. These are pin-holes or craters sometimes occurring in glazed non-vitreous ceramics while they are in the decorating kiln. The cause of this defect is the evolution of water vapour, adsorbed by the porous body, during the period between the glost firing and the decorating firing, via minute cracks in the glaze.The Causes of Pinholes in Vitreous China Sanitaryware. M E Twentyman, P Hancock. The British Ceramic Research Association. RP. 706. 1979."The A.T.Green Book." Pg. 255-268 "Spit-out." W.T.Wilkinson, A.Dinsdale. The British Ceramic Research Association. 1959.. References External links *A study of Glaze expansion Category:Pottery Category:Ceramic art Category:Ceramic engineering Category:Ceramic glazes Categoría:Defectos de los esmaltes Categoría:Esmaltes